The global positioning system (GPS) is a satellite-based radio-navigation system built and operated by the United States Department of Defense. The system uses twenty-four satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. It is possible to have more than twenty-four satellites due to the presence of some spare satellites in the GPS constellation. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks on to this signal and extracts the data contained in it. Using signals from sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
The GPS receivers can operate in many modes. In a “hot start” mode, the receiver already has the time, its last position, and the information on satellite position (also known in the art as almanacs or ephemeris) stored in its memory. The receiver can use this stored information to determine which satellites are probably visible, and it can then lock on those satellite signals in a short time. On the other hand, the receiver may have no prior data on its position, time, or almanacs stored. In this “cold start” mode, the receiver has to search for signals from all of the satellites present in the constellation. There are some other modes where partial information on time, position and almanacs are available and corresponding start mode is known as “warm start.”
The GPS receiver has to acquire and lock on to at least four satellites in order to derive the position, velocity and time. Usually, a GPS receiver has many parallel channels, each receiving signals from a separate visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of frequency and the PN code phase. Each satellite transmits a unique PN code which repeats every millisecond. The receiver locally generates a replica frequency and a replica code phase and correlates these with the received satellite signals. The PN code has to be searched in at least 2046 phases and the frequency search depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result due to local oscillator instability.
When the satellite signal is strong the receiver can detect the presence of a satellite signal in a short time. But when the signal is weak a long correlation time is needed and the integration or correlation needs to be coherent which requires large computation load. Signals may be weak due to an obstruction by foliage or buildings, or if the receiver is operating indoors. Special techniques are required to acquire the signal under the above mentioned conditions. One of the more widely used technique is known as assisted GPS (AGPS), disclosed in U.S. Pat. No. 5,884,214. This technique is mainly used with GPS receivers in cell phones. In this method a cellular base station or server provides the ephemeris, time and data bit edge position to the GPS receiver in the cell phone so that it may acquire the satellite signal. This technique requires synchronization with the base station or server, and the service has to be provided by the cell phone operator. Consequently, it results in extra subscription charges and base station augmentation.
Due to the disadvantages with AGPS, it is desirable to be able to acquire weak GPS signals without outside assistance. Examples of this approach are disclosed in U.S. Pat. Nos. 5,271,034, 6,392,590, and 6,611,756. Most of these techniques, however, are not suitable when the signal is extremely weak due to the large computation involved in carrying out lengthy integrations and fast Fourier transforms (FFTs). In these techniques, the integration involves the summing of one-millisecond correlation values. A correlation value is obtained by comparing the samples of input signal with locally available PN code samples over a one-millisecond interval. The difference between the agreement and disagreement of the samples is the correlation value. In the case of perfect correlation and no noise, the correlation value is equal to the number of samples in the one millisecond length, e.g., if the samples per code-length of one millisecond is 2046, then the perfect correlation value is 2046. But if the codes are not aligned this value may be ±130 or −2. Thus, in this case the detection of the received signal can be determined easily. In the presence of noise, however, the correlation value may not be 2046, but may have a lower value, and when the signal is extremely weak it may not be able to determine the correct correlation. In these circumstances, the receiver can estimate the correlation value over several consecutive milliseconds to arrive at a reasonable value. This summing up over several milliseconds is also known as coherent integration. The coherent integration requires that there are no sample reversals due to the residual carrier frequency. If there are reversals due to carrier frequency, the correlations may be carried out over non-reversed parts of the sample lengths and may be added by squaring each part. This is known as non-coherent integration. Compared to non-coherent integration, coherent integration provides better results for the same total length. But coherent integration requires very low residual carrier frequency and high computational load. Furthermore, in many cases the receiver processor may not be able to meet the computational requirements of coherent integration. In many cases the received GPS signals may have power ranges from strong to extremely weak, and the receiver may use same algorithm for all the signal strengths with the associated inefficient use of computational power.
Accordingly, there is a need in the art for a GPS receiver that is able to acquire strong, weak, and extremely weak GPS signals in standalone mode while not demanding a large computational load.